UNIVERSITY     OF     CALIFORNIA      AGRICULTURAL  EXPERIMENT  STATION 

BENJ.    IDE    WHEELER,    PRESIDENT 

COLLEGE    OF   AGRICULTURE  thomas  forsyth   hunt,  dean  ano  director 

H 


BERKELEY 


VAN    NORMAN,    VlCE-Di  rector  and   Dean 
University    Farm    School 


CIRCULAR  No.  144 

(December,  1915) 

OIDIUM  OR  POWDERY  MILDEW  OF  THE  VINE 

By  FREDERIC  T.  BIOLETTI  and  F.  C.  II.  FLOSSFEDER 


The  Oidium,  usually  called  Mildew  on  the  Pacific  Slope,  is  the  only 
serious  fungous  disease  of  the  vine  in  California.  It  has  been  found  in 
every  vineyard  section.  In  some,  the  cooler  and  moister  districts,  it 
does  great  damage  every  year  to  all  vines  not  properly  treated;  in 
others,  the  hotter  and  drier  districts,  it  does  little  damage  except  to  very 
susceptible  varieties  and  in  years  when  the  weather  is  favorable  to  its 
development. 


Fig.  1. — White  patches  of  Oidium  on  upper  surface  of  young  leaf. 


The  generally  recognized  standard  treatment  for  the  Oidium  is  to 
dust  the  vines  with  sulfur.  Many  growers  find  this  treatment  perfectly 
efficacious  and  satisfactory.  Many,  on  the  other  hand,  complain  that 
it  is  not  sufficient  or  even  that  it  is  useless.  In  consequence,  many 
other  methods  of  treatment  are  being  suggested  and  tried. 


The  observations  of  the  station  tend  to  show  that  the  causes  of 
failure  are  two.  First,  the  treatment  is  not  properly  applied.  The 
sulfur  used  is  sometimes  of  poor  quality  and,  very  often,  the  time 
and  method  of  application  are  ill  chosen  and  defective.  Second,  other 
diseases  and  troubles  of  the  vine  for  which  sulfur  is  useless  are  often 
mistaken  for  mildew  and  the  treatment  is  condemned  when  it  fails  to 
cure  or  prevent  these  diseases. 

The  other  methods  of  treatment  suggested  are  of  two  classes :  ( 1 ) 
Winter  spraying  of  the  dormant  vines  and  (2)  Liquid  sprays  in  the 
growing  season  with  various  fungicides. 

All  these  methods  have  been  tested  by  many  investigators  with  the 
practically  uniform  result  that  they  have  been  condemned  as  useless 
or  as  inferior  to  dry  sulfuring.  An  account  of  some  of  the  evidence 
leading  to  this  conclusion  is  given  in  Bulletin  186  of  this  experiment 
station — "The  Oidium  of  the  Vine."     Dissatisfaction  with  sulfuring 


Fig.  2. — Blackening  of  canes  due  to  Oidium. 

and  attempts  to  discover  something  better  continue,  however,  and 
some  comparative  tests  were  made  last  year  at  Davis  with  some  of  the 
remedies  most  talked  about. 

The  results  may  be  stated  briefly  as  follows: 

1.  Winter  spraying  is  useless. 

2.  Liquid  sprays  are  more  costly  and  less  efficient  than  dry 

sulfuring. 

3.  Dry  sulfuring  properly  applied  is  efficient. 

COMPARATIVE   TESTS   OF   SULFURING,   LIQUID    SPRAYS   AND 
WINTER    TREATMENT 

Sulfuring. — A  good  commercial  brand  of  sulfur  was  used  and  ap- 
plied with  efficient  knapsack  bellows.  The  work  was  done  as  nearly 
as  possible  as  it  would  be  done  in  practice  by  an  energetic  and  intelli- 
gent man.  Owing  to  the  small  size  of  the  treated  plots,  the  work  was 
done   a  little  more   rapidly   than   it   would   be   on   a   large  vineyard. 


A  test  cm  a  15  acre  vineyard  indicates  that  the  labor'  cost  would  be 
about  50  per  cent  higher  than  is  shown  in  the  experiments,  but  that,  the 
amount  of  sulfur  used  would  be  the  same.  This  indicates  that  the 
total  cost  wrould  be  about  25  per  cent  greater,  as  the  labor  and  sulfur 
costs  were  about  equal. 

Liquid  sprays. — Many  liquid  sprays  have  been  used  in  the  attempt 
to  control  oidium.  The  most  successful  have  been  those  whose  result 
is  to  leave  fine  particles  of  sulfur  on  the  leaves.  Some  of  the  type  of 
the  lime  sulfur  mixture,  contain  polysulfides  which  break  up  after  they 
reach  the  leaves  and  leave  a  residue  of  finely  divided  sulfur.  Others 
are  simply  "wetable  sulfurs"  or  finely  ground  sulfur  mixed  with 
small  quantities  of  inert  matters  which  cause  the  sulfur  to  mix  easily 
with  water  and  to  spread  and  adhere  to  the  leaves.  A  spray  of  the 
latter  type  was  used  and  applied  with  a  good  knapsack  sprayer. 

Winter  Treatment. — The  idea  of  winter  treatment  is  to  destroy 
any  spores  or  other  resting  forms  of  the  fungus  which  may  remain 
on  the  dormant  vine  after  the  fall  of  the  leaves.  One  of  the  most 
effective  sprays  for  this  purpose  and  that  most  frequently  recommended 
and  used  by  grape  growers  is  a  simple  water  solution  of  blue  stone 
(copper  sulfate).  A  2  per  cent  solution  was  used  and  applied  very 
thoroughly  with  a  Bean  powrer  spraying  machine  just  before  the  swell- 
ing of  the  buds  in  spring. 

Test  Plot. — The  experiments  were  conducted  on  a  block  of  1078 
vines  planted  6  X  12  feet  (49  rows  of  22  vines  each)  in  a  well  isolated 
vineyard  of  about  3%  acres.  The  arrangement  of  the  tests  and  the 
comparative  injury  from  mildew  are  shown  in  the  following  list : 


Mildew 


None 


Slight 


Bad 


Very  bad 


Bow 
] 

o 

3 
4 
5 
6 


8 

9 

10 

\12 

13 
11 
15 


Variety 
Burger 
Burger 
Burger 
Burger 
Burger 
Burger 

Burger 
Burger 
Burger 
Burger 

Carignane 

Carignane 

Carignane 
Carignane 
Carignane 


Treatment 


Dry  Sulfur 
Dry  Sulfur 
Dry  Sulfur 
Dry  Sulfur 
Dry  Sulfur 
Dry  Sulfur 


Wet  Sulfur 
Wet  Sulfur 
Wet  Sulfur 
Wet  Sulfur 

Wet  Sulfur 
Wet  Sulfur 


Winter  Spraying 
Winter  Spraying 


Winter  Spray 
Winter  Spray 

Winter  Spray 
Winter  Spray 
Winter  Spray 


Bad 


Slight 


Very 
slight 


Slight 


Slight 


None 


Mildew     Bow       Variety  Treatment 

16  Carignane        Dry  Sulfur         

17  Carignane         Dry  Sulfur         

18  Carignane        Dry  Sulfur         

19  Carignane         Dry  Sulfur         

20  Carignane         No  treatment 

21  Cornichon         No  treatment 

Dry  Sulfur          

Dry  Sulfur  

Dry  Sulfur  

Dry  Sulfur  

Dry  Sulfur  

Dry  Sulfur  

Dry  Sulfur  

Dry  Sulfur  

Dry  Sulfur  

Dry  Sulfur 

Dry  Sulfur  

Dry  Sulfur  

Dry  Sulfur         

Dry  Sulfur          

'36         Tokay                , Wet  Sulfur 

37  Tokay                Wet  Sulfur 

38  Tokay     •           Wet  Sulfur 

39  Tokay                Wet  Sulfur 

10         Tokay                Wet  Sulfur 

Wet  Sulfur 

Wet  Sulfur 

Wet  Sulfur 

Wet  Sulfur 

Wet  Sulfur 

'46         Muscat  Dry  Sulfur         

47  Muscat  Dry  Sulfur         

48  Muscat  Dry  Sulfur         

49  Muscat  Dry  Sulfur         


22 

Cornichon 

23 

Cornichon 

24 

Cornichon 

25 

Cornichon 

26 

Cornichon 

27 

Cornichon 

28 

Cornichon 

29 

Cornichon 

30 

Cornichon 

'31 

Tokay 

32 

Tokay 

33 

Tokay 

34 

Tokay 

35 

Tokay 

41 

Muscat 

42 

Muscat 

43 

Muscat 

44 

Muscat 

45 

Muscat 

The  plot  includes  five  varieties  of  various  degrees  of  susceptibility 
from  the  extremely  susceptible  Carignane  to  the  moderately  suscept- 
ible Burger. 

The  presence  of  two  untreated  rows  (20  and  21)  of  Carnignane 
and  Cornichon  near  the  middle  of  the  plot  insured  an  abundant  supply 
of  spores  to  infect  the  whole  plot. 


The  dry  and  wet  sulfur  were  so  arranged  that  the  effect  of  each 
could  be  observed  separately  and  in  conjunction  with  winter  treat- 
ment.    The  effect  of  winter  treatment  alone  was  also  tried. 

The  following  summary  gives  the  kind  and  date  of  treatment  and 
observations  on  the  amount  of  mildew  present  on  July  20,  1915.  On 
rows  marked  "badly  mildewed"  the  crop  was  practically  a  total  loss, 
on  those  marked  "slightly  mildewed"  the  loss  was  not  serious. 

Exp.  1.     Burger,  rows  1-4 — Dry  Sulfur  May   7,  May  20,  June   16,  July   7,   No 

Mildew. 
Exp.  2.     Burger,  rows  5-6 — Winter  Spray  March  15,  Dry  Sulfur,  May  7,  May 

20,  June  16,  July  7,  No  Mildew. 
Exp.  3.     Burger,  rows  7-10— Wet    Sulfur    May    7,    May    20,    June    16,    July    7, 

Slightly  Mildewed. 
Exp.  4.     Carignane,  rows  11-12 — Winter   Spray  March   15,   Wet  Sulfur  May   7, 

20,  June  16,  July  7,  Badly  Mildewed. 
Exp.  5.     Carignane,  rows  13-15 — Winter  Spray  March  12,  Very  Badly  Mildewed. 
Exp.  6.     Carignane,  rows  16-19 — Dry  Sulfur  May  7,  20,  June  15,  July  7,  Badly 

Mildewed. 
Exp.  7.     Carignane,  row  20 — No   treatment,  Badly  Mildewed. 
Exp.  8.     Comiehon,  row  21 — No  treatment,  Badly  Mildewed. 
Exp.  9.     Comiehon,    rows  22-30 — Dry    Sulfur    May    7,    20,    June    16,    July    7, 

Slightly  Mildewed. 
Exp.  10.     Tokay,  rows   31-35— Dry   Sulfur   May    7,    20,   June   16,   July   7,    Very 

Slightly  Mildewed. 
Exp.  11.     Tokay,  rows  36-40— Wet  Sulfur  May  7,  20,  June   16,  July  7,    (More 

Mildew  than  in  Exp.  10,  B.  31-35). 
Exp.  12.     Muscat,  rows,  41-45 — Wet  Sulfur  May  7,  20,  June  16,  July  7,  Slightly 

Mildewed. 
Exp.  13.     Muscat,  rows  46-49— Dry    Sulfur    May    7,    20,    June    16,    July    7,    No 

Mildew  found  except  a  very  little  on  one  bunch  of  one  vine. 


COMPARISON    OF    COST    OF    SULFURING    AND    SPRAYING    VINES 

Sulfuring,  per  Acre — 


S.  lbs.d) 

Minutes(2) 

Total  cost 

May      7 

1.72 

$0,043 

22.5 

$0,075 

$0,118 

May   20 

1.72 

0.043 

23.4 

0.078 

0.121 

June  16 

4.40 

0.110 

26.5 

0.088 

0.198 

July      7 

6.87 

0.172 

29.0 

0.093 

0.265 

Totals  14.71  $0,368  101.4  $0,334  $0,702 


Spraying,  per  Acre- 


Spray  gals.(:: 

) 

Minutes  (2  j 

Total  cost 

May      7 

7.01 

$0,063 

33.5 

$0,112 

$0,175 

May   20 

9.70 

0.087 

35.3 

0.118 

0.205 

June  16 

9.70 

0.087 

41.0 

0.137 

0.224 

July     7 

13.80 

0.124 

50.0 

0.167 

0.291 

Totals 

40.21 

$0,361 

159.8 

$0,534 

$0,895 

(1)  Sulfur  reckoned  at  $2.50  per  100  pounds. 

(2)  Time  reckoned  at  20  cents  per  hour. 

(3)  Wet  sulfur  reckoned  at  $7.50  per  100  pounds  as  purchased   or  9  <•< 
per  gallon  for  the  spray  used. 


The  vines  were  in  their  third  year,  well  grown  and  very  uniform 
in  size.     They  were  all  short  pruned  and  of  medium  height. 

The  amount  of  sulfur  used  and  the  time  taken  in  applying  it  are 
unusually  small  and  may  be  taken  as  the  minimum  for  four  sulfurings 
of  a  bearing  vineyard.  With  trellised  or  very  high  vines  even  with 
competent  workmen  the  cost  would  be  nearly  double. 

In  another  experiment  vineyard  of  13 y2  acres  at  Davis  five  sul- 
furings were  given  at  a  cost  of  $1,064  per  acre.  This  vineyard  con- 
tained several  hundred  varieties  of  vines  of  various  ages  and  sizes. 
About  75  per  cent  were  bearing  and  from  three  to  five  years  old,  the 
remainder  were  one  or  two  years  old.  Many  styles  of  pruning  were 
represented,  including  two-wire  trellises  and  high  cordons.  In  spite 
of  these  unfavorable  conditions  and  the  presence  within  a  few  hundred 
feet  of  untreated  and  affected  vines  there  was  practically  no  crop  loss 
from  mildew. 

It  is  not  uncommon  for  grape  growers  to  use  twenty,  thirty,  or  more 
pounds  of  sulfur  per  acre  at  a  cost  for  material  and  labor  of  over  $1.00 
for  each  sulfuring.  Most  of  this  sulfur  is  wasted  and  often  the  mildew 
is  not  controlled,  owing  to  defective  methods  and  inappropriate  times 
of  application. 

SUMMARY    OF    RESULTS    OF    TESTS 

1.  Four  dry  sulfurings,  i.e.,  two  before,  one  during  and  one  after 
blossoming,  controlled  the  mildew  perfectly  on  varieties  of  average 
susceptibility,  (Burger  and  Muscat),  even  when  the  conditions  were 
exceptionally  favorable  to  the  development  of  the  fungus.  The  chief 
of  these  conditions  are  a  source  of  abundant  spores  for  reinfestation 


and  suitable  weather  for  their  germination  and  growth.     (See  Experi- 
ments 1,  2  and  13.) 

The  proximity  of  two  untreated  rows  (rows  20  and  21)  of  Carig- 
nane  and  Cornichon  insured  an  abundant  and  continuous  supply  of 
spores.  The  weather  of  early  summer  was  comparatively  moist  and 
cool,  a  condition  favorable  to  the  fungus  in  the  interior  valleys.  The 
mean  temperature  at  Davis  for  the  month  of  May  was  60.7  or  7.2 
degrees  below  the  normal  and  71.2  for  June  or  3.6  degrees  below 
normal. 

2.  Under  the  same  conditions,  with  more  susceptible  varieties 
(Cornichon  and  Tokay),  the  same  treament  controlled  the  mildew 
fairly  well.     (See  Experiments  9  and  10.) 

3.  With  a  variety  of  maximum  susceptibility  (Carignane)  none  of 
the  treatments  has  any  perceptible  effect.  (See  Experiments  4,  5,  6, 
7  and  8.) 

4.  The  wet  spray  (wet  sulfur  12  pounds  in  100  gallons  of  water) 
used  was  somewhat  less  effective  than  the  dry  sulfur.  (Compare 
Experiment  1  with  3;  10  with  11;  13  with  12.) 

5.  Spraying  with  copper  sulfate  solution  just  before  the  starting 
of  the  buds  (winter  spraying)  was  unnecessary.  (Compare  Experi- 
ment 1  with  2.) 

6.  Spraying  in  winter  had  no  effect.  (Compare  Experiments  4 
and  5  with  6  and  7.) 

7.  The  cost  of  perfect  control  with  four  dry  sulfurings  was  70 
cents  per  acre;  that  of  partial  control  with  wet  spraying  90  cents  or 
28.6  per  cent  more. 

8.  The  cost  of  each  treatment  increased  with  the  lateness  of  the 
season.  Calling  the  cost  of  the  first  dry-sulfur  treatment  100  per  cent, 
the  cost  of  later  treatments  was  as  follows:  Dry  sulfur  at  blossoming 
167.8  per  cent,  after  setting  224.4  per  cent;  wet  spray  at  blossoming 
190  per  cent,  after  setting  246  per  cent. 


CONCLUSIONS 

1.  Mildew  on  most  varieties  can  be  controlled  cheaply  by  proper 
sulfuring,  even  under  unfavorable  conditions  in  the  interior  valley. 

2.  It  is  probable  that  it  can  be  controlled  on  all  varieties  by  the 
same  means  in  the  same  region. 

3.  The  presence  of  missed  or  untreated  vines  much  increases  the 
difficulty  of  controlling  the  disease  on  all  the  vines. 

4.  Winter  treatments  are  unnecessary  and  useless. 


8 


5.  Wet  sprays  are  more  expensive  to  apply,  and  that  tested  less 
effective  than  dry  sulfur.  It  is  probable  that  other  wet  sprays  would 
be  no  more  or  even  less  effective. 


DIRECTIONS  FOR  SULFURING  VINES 


Number  of  Sulfurings- 
sulfur  will  depend  on  the 


-The  number  of  times  it  is  necessary  to 
locality,  the  variety,  the  season  and  the 


Fig.  3. — Vine  at  right  stage  of  growth  for  the  first  sulfuring. 


sources  of  infection  and  will  vary  from  one  to  six.  In  the  interior 
valleys  two  will  usually  be  enough  if  the  work  is  done  regularly  and 
properly  in  all  the  vineyards. 

The  first  sulfuring  should  be  done  when  the  shoots  are  not  more 
than  six  or  eight  inches  long.  (See  Fig.  3.)  Later,  the  foliage  becomes 
dense  and  it  is  difficult  to  cover  every  part  of  the  surface  with  sulfur. 
At  the  first  sulfuring  it  should  be  possible  to  see  daylight  through  all 
parts  of  the  vine. 


The  first  sulfurings  are  preventive  and  are  the  most  important 
and  effective.  If  we  wait  until  mildew  is  perceived,  the  cost  of  control 
will  be  much  greater  and  it  will  often  be  impossible  to  control  the 
disease  completely. 

The  second  sulfuring  should  be  given  during  or  just  before  blossom- 
ing. At  this  time  the  interior  of  the  mass  of  foliage  will  be  dense 
in  places,  but  the  first  sulfuring  has  already  covered  the  parts  which 
might  escape  the  second.  With  Muscats  and  most  wine  grapes  these 
sulfurings  will  usually  give  perfect  control  if  there  are  no  summer 
irrigations  and  no  untreated  vines  or  parts  of  viues  in  the  immediate 
vicinity.  With  very  susceptible  varieties,  such  as  the  Carignane,  or 
excessively  vigorous  growers,  like  the  Emperor,  or  where  summer 
irrigation  is  practised,  a  third  sulfuring  may  be  necessary  about  the 
time  the  grapes  are  half  grown.  Only  imperfect  work  or  abundant 
reinfection  from  untreated  vines  should  necessitate  more  than  three 
treatments  except  in  the  coast  regions  of  summer  fogs  and  even  here 
three  will  usually  be  sufficient. 

Time  of  Sulfuring. — The  time  of  day  is  of  no  importance,  provid- 
ing there  is  not  so  much  wind  as  to  prevent  proper  distribution  or 
so  much  moisture  that  the  leaves  tend  to  stick  together  and  escape 
the  sulfuring.  Dry,  moderately  warm  weather  is  the  best,  but  the 
first  sulfuring  should  be  done  even  if  such  weather  does  not  occur. 

Distribution  of  the  Sulfur. — It  should  be  the  aim  in  sulfuring  to 
eradicate  absolutely  every  vestige  of  mildew  from  the  vineyard.  This 
can  be  done  only  by  placing  sulfur  on  every  exposed  surface.  If  a 
single  leaf  in  the  vineyard  escapes  it  may  produce  enough  spores  to 
reinfect  the  whole.  This  complete  distribution  is  easily  accomplished 
at  the  first  sulfuring  if  done  at  the  time  recommended.  It  is  possible 
but  more  difficult,  during  blossoming  and  practically  impossible  later. 
It  is  for  this  reason  that  the  early  sulfurings  are  the  most  important. 

Proper  distribution  can  be  accomplished  only  with  very  fine  sulfur, 
which  easily  floats  in  the  air,  and  with  a  distributing  machine  which 
throws  it  out  in  a  spreading  cloud.  The  object  should  be  to  produce 
a  cloud  of  light  sulfur  dust  which  will  float  through  the  vine  and  leave 
the  particles  of  sulfur  distributed  on  every  minute  piece  of  surface. 
This  cannot  be  done  with  can  shakers  or  sulfur  bags,  except  by  the 
use  of-  unnecessarily  large  amounts  of  sulfur.  Some  form  of  blower 
which  will  thoroughly  break  up  the  adhering  masses  of  sulfur  particles 
and  expel  them  with  sufficient  force  is  needed.  Suitable  machines  are 
obtainable  in  California,  but  all  in  use  are  of  foreign  manufacture. 


10 

Kind  of  Sulfur. — The  finer  the  sulfur  the  more  perfect  the  distri- 
bution. Most  of  the  sulfurs  which  have  been  offered  to  the  grape- 
growers  during  late  years  are  of  good  quality,  judging  by  the  samples 
received  at  the  station.  Some  have  a  considerable  amount  of  coarse 
material  which  may  be  considered  waste.  Lately  some  exceedingly 
fine-ground  sulfurs  have  been  produced  which  are  undoubtedly  excel- 
lent. Some  sulfurs,  while  fine,  have  a  tendency  to  form  lumps  which 
are  difficult  to  break  up  and  are  thrown  out  entire  by  some  machines. 
Such  lumps  are  completely  wasted  and  increase  the  amount  of  sulfur 
needed  considerably. 

Examination  of  Sulfur. — The  grower  can  easily  determine  the 
suitability  of  a  sulfur  by  a  few  simple  tests. 

1.  A  very  good  idea  of  the  fineness  of  a  sulfur  can  be  obtained 
by  means  of  an  ordinary  good  hand  magnifying  glass,  costing  about 
$1.00.  A  half-inch  Coddington  lens  is  suitable.  "With  a  little  experi- 
ence sublimed  can  be  distinguished  from  ground  sulfur  by  this  means. 
The  particles  of  sublimed  sulfur  are  spherical  and  with  smooth  outlines ; 
those  of  ground  sulfur  are  irregular  and  with  angular  outlines. 

The  sulfur  is  most  easily  examined  when  spread  between  two  pieces 
of  glass  (microscopical  slides)  and  held  over  a  dark  background.  A 
minute  pinch  of  the  sulfur  is  placed  on  one  piece  of  glass  and  spread 
out  by  rubbing  it  with  the  other.  It  spreads  more  perfectly  if  first 
moistened  with  a  little  ether  or  gasoline. 

2.  With  a  fine  sulfur  there  should  be  little  or  no  grit  felt  when 
it  is  rubbed  on  the  palm  of  the  hand. 

3.  If  the  sulfur  is  suborned  the  deeper  yellow  the  color  the  finer  it 
is.     Ground  sulfur,  on  the  contrary,  is  lighter  or  whiter  the  finer  it  is. 

4.  The  finer  the  sulfur  the  more  bulky.  With  sacks  of  equal  weight 
the  larger  will  contain  the  better  sulfur. 


11 


Fig.  4. — Sublimed  Sulfur  of  very  good  quality 


Fig.  5. — Sublimed  Sulfur  of  poor  quality. 


12 


1*^1 


■&&.T4*'** 


?#V> 


Fig.  6. — Ground  Sulfur  of  good  quality. 


Fig.  7. — Ground  Sulfur  of  very  bad  quality 


